Scanning systems in modern day apparatus are becoming more accurate on the one hand but more complicated and expensive on the other hand. In certain optical image systems, a collimated source of light, as from a laser and associated optical lenses, impinges on a rotating polygon scanner which by the rotational movement thereof causes the reflected light to revolve about an axis near the center of rotation of the rotating polygon. The reflected light can be utilized to scan a document at the input end of an imaging system or can be used to impinge upon a photosensitive medium, such as a xerograhic drum, in the output mode. Many systems have been disclosed in the art to overcome various optical and other distortions in the optical flying spot imaging system.
For example, in U.S. Pat. No. 4,247,160, entitled "Scanner With Reflective Pyramid Error Compensation", issued to the same inventor and assignee as the present application, a laser beam scanner is disclosed having a positive cylinder mirror located between the polygon scanner and the photosensitive surface. The positive cylinder mirror, which has power in the cross scan plane but no power in the scan plane itself, minimizes scan to scan spot position errors due to angular alignment errors between adjacent facets of the polygon scanner without introducing significant cross scan field curvature. Residual cross scan field curvature is compensated for by providing a negative refractive cylinder lens with power in the cross scan plane between the cylindrical mirror and the polygon scanner.
Another patent application, Ser. No. 032,985, filed Apr. 25, 1979, entitled "Post-Polygon Objective Scanner", by the same inventor and assignee as the present application, now abandoned, discloses a scanning system in which the facets of a rotating polygon are illuminated by a collimated light beam, and a post-polygon objective, comprised of one negative lens element near the polygon and one positive lens element, is utilized to focus the monochromatic light beam at a photosensitive surface. The system provides a flat field of focus at the photosensitive surface, provides a constant linear scan rate across the photosensitive surface, eliminates bow distortion, and eliminates sensitivity to polygon facet alignment and fabrication errors, and has diffraction-limited performance.
According to the present invention, it was discovered that the positive cylinder mirror, which has power in the cross scan plane but no power in scan plane, as disclosed in the aforementioned U.S. Pat. No. 4,247,160, when utilized with the teachings of the aforementioned Ser. No. 032,985, specifically the teachings of a negative lens and positive lens, as one lens herein, provides a flat field of focus at the photosensitive surface with a constant linear scan rate across the surface.